Preparation Of PVDF-HFP-based Photothermal Membrane And Its Application In Solar-driven Membrane Distillation

Membrane distillation(MD)is a desalination technology based on the membrane thermal evaporation process.Water vapor generated on the surface of the feed side is diffused through the membrane pores to the other side of the membrane,which is condensed to form fresh water.It is considered to be the most promising desalination technology due to its high desalination rate,low cost,and modular design.However,for the traditional membrane distillation system,its inherent temperature polarization phenomenon(the temperature of the membrane surface on the feed side is lower than the temperature of the upper feed liquid)has hindered its large-scale industrial application.In recent years,solar energy has been introduced as a heat source to heat feed solutions in traditional MD technology,giving rise to a new type of desalination technology Solar-driven membrane distillation(SDMD).This technology uses photothermal membrane for in-situ heating,which not only effectively eliminates temperature polarization,but also reduces the cost of desalination,so it is a promising seawater desalination technology.However,SDMD currently faces some key problems:(1)low conversion efficiency of light to heat;(2)high cost of photothermal materials and complicated preparation process;(3)serious long-term pollution on the membrane.In view of these challenges,this thesis aims to develop high-performance membranes for SDMD,by using the PVDF-HFP membrane as the substrate,and modifying the surface of the membrane by suction filtration and deposition method to obtain PVDF-HFP-based photothermal membranes.The structure of the photothermal membrane and its desalination performance in the SDMD system were systematically examined.The specific research contents are as follows:(1)A photothermal membrane for SDMD was prepared by coating Fe₃O₄ nanoparticles on the surface of PVDF-HFP nanofibrous membrane.Fe₃O₄/PVDF-HFP photothermal membrane can realize local heating in situ under the irradiation of a simulated solar source,which increases the temperature of the membrane surface and eliminates the temperature polarization effect.Due to the high full-spectrum absorbance and excellent light-to-heat conversion ability of the Fe₃O₄ nanomaterial,coupled with the high porosity of the PVDF-HFP membrane,the photothermal membrane showed a permeate flux of 0.97 kg m^-2h^-1 with a salt rejection rate of 99.99%under 1 k W m^-2 solar irradiation,and the photothermal conversion efficiency is among the highest(53%)reported so far.Furthermore,such a composite membrane worked stably in a pilot-scale system and demonstrated a 21.99 kg m^-2h^-1 which is 11%higher than the solar-free system.(2)Highly graphitized carbon spheres(GCS)were prepared using dopamine as a carbon source and was used as a photothermal material.The prepared GCS was dispersed in a Nafion solution,and suction-filtered onto the surface of the PVDF-HFP membrane to obtain a superhydrophobic surface.The other side of the membrane was subjected to hydrophilic modification by self-polymerization,yielding a dopamine-based Janus structural membrane with totally different hydrophilicity on the two sides.The hydrophobic side of the Janus membrane can prevent the membrane from being wetted and contaminated,improve the long-term service performance of the membrane.As such the hydrophilic design of Janus membrane eliminated the air gap between the membrane and the solution,reduced the transmission distance of water vapor,speeded up the condensation process of water,and effectively eliminated the temperature polarization and improved the performance of membrane distillation,superimposed with the excellent photothermal conversion capability of GCS.The Janus membrane showed a high permeation flux(1.29 kg m^-2 h^-1)in the SDMD system under 1 k W m^-2 solar irradiation.The performance was 396%higher than that of the PVDF-HFP membrane,the salt rejection rate was 99.99%,and the light-to-heat conversion efficiency was 60%.